A tsunami (pronounced sue-nahm-ee) is a series of
huge waves that can cause great devastation and loss of life when they strike a
coast.

Tsunamis are caused by an underwater earthquake, a volcanic
eruption, an sub-marine rockslide, or, more rarely, by an asteroid
or meteoroid
crashing into in the water from space. Most tsunamis are caused by underwater
earthquakes, but not all underwater earthquakes cause tsunamis - an earthquake
has to be over about magnitude 6.75 on the Richter
scale for it to cause a tsunami. About 90 percent of all tsunamis occur in
the Pacific Ocean.

Many tsunamis could be detected before they hit land, and the loss of life
could be minimized, with the use of modern technology, including seismographs
(which detect earthquakes), computerized offshore buoys that can measure changes
in wave height, and a system of sirens on the beach to alert people of potential
tsunami danger.

NOTE: If you see the water recede quickly and unexpectedly from a
beach (this is called drawback), run toward higher ground or inland -- there may
be a tsunami coming. Also, if you are on the coast and there is an earthquake,
it may have caused a tsunami, so run toward higher ground or inland. Some
beaches have tsunami warning sirens -- do not ignore them. The first wave in a
tsunami is often not the largest; if you experience one abnormally-huge wave, go
inland quickly -- even bigger waves could be coming soon.

The Word
Tsunami:
The word tsunami comes from the Japanese
word meaning "harbor wave." Tsunamis are sometimes incorrectly called
"tidal waves" -- tsunamis are not caused by the tides
(tides are caused by the gravitational force of the moon on the sea). Regular
waves are caused by the wind.

The Development of a Tsunami:
A tsunami starts when a huge volume of water is quickly shifted. This rapid
movement can happen as the result of an underwater earthquake (when the sea
floor quickly moves up or down), a rock slide, a volcanic eruption, or another
high-energy event.

After the huge volume of water has moved, the resulting wave is very long
(the distance from crest to crest can be hundred of miles long) but not very
tall (roughly 3 feet tall). The wave propagates (spreads) across the sea in all
directions; it can travel great distances from the source at tremendous speeds.

The Size of a Tsunami:
Tsunamis have an extremely long wavelength (wavelength is the distance between
the crest (top) of one wave and the crest of the next wave) -- up to several
hundred miles long. The period (the time between two successive waves) is also
very long -- about an hour in deep water.

In the deep sea, a tsunami's height can be only about 1 m (3 feet) tall.
Tsunamis are often barely visible when they are in the deep sea. This makes
tsunami detection in the deep sea very difficult.

The Speed of a Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast
as a jet flies. It can take only a few hours for a tsunami to travel across an
entire ocean.
A regular wave (generated by the wind) travels at up to about 90 km/hr.

As a tsunami wave approaches the coast (where the sea becomes shallow), the
trough (bottom) of a wave hits the beach floor, causing the wave to slow down,
to increase in height (the amplitude is magnified many times) and to decrease in
wavelength (the distance from crest to crest).

In addition to large tsunami waves that crash onto shore, the waves push a
large amount of water onto the shore above the regular sea level (this is called
runup). The runup can cause tremendous damage inland and is much more common
than huge, thundering tsunami waves.

Tsunami Warning Systems:
Tsunami warning systems exist in many places around the world. As scientists
continuously monitor seismic activity (earthquakes), a series of buoys float off
the coast and monitor changes in sea level. Unfortunately, since tsunamis are
not very tall in height when they are out at sea, detection is not easy and
there are many false alarms. Sirens at affected beaches may be activated -- do
not ignore them!

Regular waves (caused by the wind) are very different from tsunami waves.
Tsunami waves are much faster than wind-generated waves and they have a much
longer wavelength (the distance from crest to crest). In the deep sea, tsunami
waves are very small, but by the coast, they dwarf regular waves.

How Often do Tsunamis Occur?
Tsunamis are very rare. There are roughly six major tsunamis each century.